This invention relates to a helical scan magnetic playback typical of video tape recorders. In particular, the invention relates to a large transverse correction of the rotary drum, either for start-up conditions or to compensate for fast tape movement.
In a conventional 1/2 inch video tape recorder, a magnetic tape is wound around an angle of 180 degrees on a rotary drum having two rotary heads A and B mounted 180 degrees apart from each other so as to record video data by alternately selecting one of the two rotary heads A and B using a switching pulse (SWP). The rotary drum rotates with a sliding movement with respect to the inclined tape. In that case, there is formed a track for each field, the track being tilted to the travel direction of the magnetic tape. On the other hand, audio data is recorded on a track in parallel with the travel direction of the magnetic tape or the tilted track in combination with video data after the video data has been subjected to frequency modulation.
In the recent so-called standardized 8 mm video tape recorder, a magnetic tape is wound instead around an angle of 221.degree. on a rotary drum to record ordinary video data in its 180.degree. area, whereas digital (PCM) audio data is recorded in the 36.degree. area (the remaining 5.degree. area being a margin for video data). As shown in FIG. 1, video and audio areas are formed on the tilted track of the magnetic tape in such a 8 mm video tape recorder. In the PCM area, there is recorded coded data (location data) for retrieving the initial point of a numbered program or of the magnetic tape, the elapsed time (hour, minute, second) from the initial point of the program, recording date, etc. in addition to the audio signal in the form of PCM (pulse code modulated) data. In consequence, it is possible to retrieve a given location (program) on a magnetic tape by reading the code.
When a given location is to be retrieved, the magnetic tape is caused to travel at high speed in order to shorten the retrieval time. As shown in FIG. 1, for instance, while the rotary head (A or B) traces the magnetic tape once from its lowermost edge to its uppermost edge when the tape is rewound at a speed n times higher than the normal playback speed, the rotary head is expected to traverse (n+1) (1+36/180) tracks ((n-1) (1+36/180) tracks in the case where the tape is moved in the fast forward mode). Since the azimuth conforms on every other track, the envelope of the RF signal of a playback signal becomes what is shown in FIG. 2. In other words, the playback signal level becomes higher on the track where the azimuth conforms to what is intended, whereas the level becomes lower on the track where the azimuth does not conform. When the playback signal level becomes low, noise will increase, thus causing the code to be unreadable. Accordingly, the disadvantage is that the time required for retrieving is long in the conventional apparatus because it is impossible to set a significantly increased travel speed during retrieving. It has been possible to achieve a fast mode speed several times higher than the playback speed at the most.
FIG. 3 is a block diagram of a conventional helical scan magnetic playback. In FIG. 3, piezo-electric elements 3 and 4 are the driving means for driving the rotary heads 1 and 2 and installed 180 degrees apart from each other. Amplifier circuits 45 and 46 amplify the signals from the rotary head 1 and 2 and apply them to a switch circuit 47, respectively. The switch circuit 47 operates to alternately select one of the signals from the rotary heads 1 and 2 corresponding to head switching signals. The output from the switch circuit 47 is supplied to the necessary circuit (not shown) where it is demodulated and displayed on a CRT and so on (not shown). Then it is supplied to a low-pass filter 8 where a pilot signal for tracking is separated and extracted. In other words, pilot signals f.sub.1 to f.sub.4 having different frequencies are successively recorded on each track of a magnetic tape. The frequency of each of the pilot signals f.sub.1 to f.sub.4 ranges from, for instance, 6.5f.sub.H, 7.5f.sub.H, 10.5f.sub.H up to 9.5f.sub.H (f.sub.H being the frequency of horizontal synchronizing signal). A reference pilot signal generating circuit 10 is used to successively generate a reference pilot signal having the same frequency as that of the pilot signal f.sub.1 to f.sub.4. A mixer 9 multiplies the pilot signal from the low-pass filter 8 by the reference pilot signal from the reference pilot signal generating circuit 10 and outputs the best component thereof. A fundamental band-pass filter 11 and an harmonic band-pass filter 12 respectively separate the signal having a frequency of f.sub.H and the signal at 3f.sub.H and passes them. Accordingly, the beat component of the fundamental frequency f.sub.H and that of the harmonic frequency 3f.sub.H are detected by detectors 13 and 14, whereas the difference between them is produced by a differential amplifier 15. A tracking error signal is thus generated.
The tracking error signal is compared with the output of a D/A converter circuit 19 by a comparator 16. An adder 17 adds+1 when, for instance, the tracking error signal is greater than the output of the D/A converter circuit 19 and -1 when it is smaller than the output thereof and supplies the incremented or decremented sum to be stored in the shift register 18. The shift register 18 supplies the added signal to the D/A converter circuit 19 and the adder 17 after a predetermined time has elapsed, for instance, the time equivalent to one frame. The signal supplied to the D/A converter circuit 19 is subjected to digital to analog conversion and supplied both back to the comparator 16 and to a low-pass filter 24. The low-pass filter 24 smooths the D/A converted signal and supplies the signal to a driving circuit 6. The driving circuit 25 amplifies the signal and outputs it to the piezo-electric element 3 as a high voltage signal. On the other hand, the signal supplied from the D/A converter circuit 19 to the comparator 16 is again compared with the tracking error signal produced by the differential amplifier 15 and +1 or -1 is added to the signal supplied by the shift register 18 to the adder 17 depending on the result compared and the added signal is again supplied and stored in the shift register 18. This operation is repeated and so controlled that the transverse position of the rotary head can correctly trace the track. In the same manner, the comparator 20, an adder 21, a shift register 22, a D/A converter circuit 23, a low-pass filter 26 and a driving circuit 7 are provided to rotate another rotary head 2 using a piezo-electric element 4. However, clocking signals control the updating of the shift registers 18 and 22 so that they are alternately updated and so that the piezo-electric elements 3 and 4 altenatively drive the rotary heads 1 and 2.
To provide tracking control for the rotary heads 1 and 2 corresponding to the tracking error signal one frame prior, the shift registers 18 and 22 cause the response speeds of the piezo-electric elements 3 and 4 to be delayed. This is because accurate tracking control will be unavailable to the separate heads if the output of the differential amplifier 15 is directly supplied to the driving circuits 6 and 7.
The conventional apparatus is so arranged as to make the tracking error signal ultimately converge by incrementing or decrementing the output of the shift register 18 or 22, one after the other. Consequently, if 8-bit data is to be stored in the shift register 18 or 22, a lengthy time (128/30=4.2 seconds) will be required to correctly trace tracks by means of the rotary heads 3 and 4. This is disadvantageous.